1. Field of the Invention
The present invention relates to an exposure apparatus or aligner for exposing an exposure body to a pattern on an original body, for example, a semiconductor circuit, a liquid crystal TV panel circuit, or the like, and in particular, to such an aligner which is suitable for division exposure wherein a large panel, for example, is exposed in a step-and-repeat manner, thereby transferring images of the pattern on the original body onto different regions on the exposure body with a proper exposure accuracy.
2. Related Background Art
In a mirror projection type semiconductor exposure apparatus, a mask and a substrate or wafer are put on a carriage, and the wafer is exposed to an entire pattern on the mask with radiation by scanning the mask and the wafer relative to a mirror projection system and an illuminating system.
However, in recent years, the diagonal length of the wafer has been enlarged for the purposes of reducing the cost of chips and a need has arisen to fabricate a large-sized liquid crystal display panel for liquid crystal TVs and the like, and thus the pattern image plane has been made large in size. Therefore, needs have been brought forth to augment the range of exposure and to extend the scan length, so that the size of exposure apparatuses has inevitably been made large.
To cope with this tendency, a step-and-scan or step-and-repeat exposure system has been presented for dividing the image plane on the wafer into a plurality of different imaginary areas and conducting a scan-exposure of each the different areas on the wafer to the pattern on the mask each time a different area on the wafer is stepwise moved under an exposure station.
Referring to FIG. 1, a first prior art exposure apparatus wil be explained which was has been presented as such a step-and-repeat type aligner (this will be referred to as a first prior art apparatus hereinafter). In FIG. 1, designated by reference numeral 1 is a photomask on which an exposure pattern or print image is formed, and the mask 1 is carried on a mask stage 2 which is movable in the X-, Y- and .theta.-directions. The X- and Y-directions are two rectilinear directions which are orthogonal to each other, while the .theta.-direction is a rotational direction in a plane containing the X- and Y-directions. Designated by reference numeral 3 is a glass substrate or plate on which multiple pixels and switching transistors for on-off control of these pixels are to be formed by a conventional photolithographic process for constructing a liquid crystal display panel and which has a rectangular square shape whose diagonal length is about 14 inches. The substrate 3 is carried on a substrate stage 4 which is movable in the X-, Y- and .theta.-directions. The stepwise movement of the substrate stage 4 is controlled by a precise length-measuring system which uses laser receivers 61, 62 and 63 of a laser interferometer, as shown in FIG. 6.
Further, in FIG. 1, denoted by reference numeral 5 is a conventional mirror projection optical system which includes a combination of concave and convex mirrors and, at one-to-one magnification, projects on the substrate 3 the pattern image of the mask 1 that has been brought to a prescribed position or exposure station by the mask stage 2. Designated by reference numeral 6 is an illumination optical system for illuminating the mask 1 in the exposure station with radiation of a given wavelength from a light source (not shown), and this illumination system 6 exposes a photosensitive layer on the substrate 3 to the pattern on the mask 1 to transfer this pattern onto the substrate 3. The optical axis of the projection optical system 5 is aligned with that of the illumination system 6.
Further, in FIG. 1. there are provided linear air bearing (LAB) assemblies 7 and 7 which are respectively slidable along two rail guides 8 and 8 formed in the Y-direction, and one of which (right one) is an X- and Z-direction constraint type (i.e., constrained with respect to movement in the X- and Z-directions) and the other (left one) is a Z-direction constraint type. The LAB's 7 and 7 support a holder or carriage 9 for carrying the mask stage 2 and the substrate stage 4 maintaining a prescribed relationship therebetween, so as to make it possible to move the mask 1 on the mask stage 2 and the substrate 3 on the substrate stage 4 as a unit.
Finally, denoted by reference numeral 11 is a mask feeding apparatus for sequentially feeding the masks 1 to the mask stage 2, and denoted by reference numeral 12 is a gap sensor for detecting the gap between the focal plane of the projection system 5 and the surface of the substrate 3, which is, for example, an air micrometer, or a photoelectric type sensor for detecting such a gap using light reflected from the substrate 3, and denoted by reference numeral 13 is a base member for supporting the exposure system 5, illumination system 6 and guide rails 8 and 8 in a predetermined interrelationship.
In the first prior art apparatus of FIG. 1, the surface of the substrate 3 is divided into, e.g., four imaginary exposure regions or parts, and these exposure regions are sequentially brought to the exposure station under the mask 1 and the projection system 5 by the stepwise movement of the substrate stage 4. Thus, the exposure of the mask pattern is conducted four times on the four different regions of the substrate 3, and the pattern of the liquid crystal display panel corresponding to one layer thereof is printed on the whole surface of the substrate 3. The carriage 9 carries the substrate stage 4 together with the mask stage 2 in order to scan the mask 1 and substrate 3 quickly and precisely relative to the projection system 5 and illumination system 6 and conduct the step-and-scan exposures.
As shown in FIG. 6, an L-shaped or right-angled mirror 43 (hereinafter referred to as a square) is mounted on a .theta.-table 42 of the substrate stage 4, and the distances to the square 43 in the X-and Y-directions are measured by laser interferometers 61, 62 and 63 to monitor the X and Y coordinates of the substrate stage 4 and substrate 3 when the substrate 3 is moved stepwise.
In the first prior art apparatus, however, there exists a disadvantage that such a shift or difference as is shown in FIG. 7 may occur between the individual patterns which have been printed on the substrate 3 in the step-and-repeat manner, especially at the time of pattern transfer of a first mask onto a first layer of the substrate 3. This is because no alignment mark has yet been formed on the substrate 3 so that alignment of the substrate with respect to the first mask is not attainable. On top of that, the laser interferometer measures a relative distance from a certain reference position and the position of the square 43 at the time of switching on the power source or the start of the exposure apparatus is usually set as this reference. As a result, the above shift or difference is likely to occur.
In more detail, if the square 43 is inclined by an angle .theta. relative to the X and Y slide axes of the substrate stage 4 (XY table 44 shown in FIG. 3) as shown in FIG. 6, the square 43 moves from a position depicted by a solid line to a position depicted by a broken line when the substrate stage 4 has been moved by a distance L in the X-direction based on the values measured by the receivers 61 and 62 of the laser interferometer. But, at this time, the value measured by the receiver 63 will deviate from a correct value by .DELTA.Y=L.multidot.tan .theta., and hence the substrate stage 4 will erroneously be moved by a distance -.DELTA.Y in the Y direction by an unshown substrate stage driving circuit in order to correct the position of the substrate stage 4 deviating by a distance .DELTA.Y in the Y direction. Likewise, when the substrate stage 4 has been moved in the Y direction, the position of the substrate stage 4 in the X direction will fluctuate by an the amount .DELTA.X. The above first prior will art apparatus is disclosed in the U.S. Pat. No. 4,814,830 (Isohata et al.; issued Mar 21, 1989).
To overcome this problem, there has been proposed an exposure apparatus or aligner (referred to as a second prior art apparatus hereinafter) as follows. In this second prior art apparatus, at the time of switching on the power source or at any desired time, the inclination of the square 43 mounted on the stage will be detected relative to the X and Y slide axes, and when the stage is moved, the amount of displacement in the X and Y directions is to be corrected according to that detected inclination to improve the stage feed accuracy. This second prior art apparatus is disclosed in the U.S. Pat. No. 4,676,630 (Matsushita et al.; issued Jun 30, 1987).
In the second prior art apparatus, at the time of switching on the power source or at any desired time the inclination of the square 43 is measured from the amount of movement of the XY stage 44 in one direction and the amount of displacement in a direction orthogonal to this one direction, and the XY stage 44 is stepwise moved or fed based on the result of those measurements.
However, in this system, when the reproducibility of the moving accuracy of the stage 44 in a yawing direction is poor, an error occurs in the measurement of the square's inclination, corresponding to the reproducibility. Thus, the difference or shift between pattern images still occurs, as shown in FIG. 7.
In general, the reproducibility of the movement accuracy of the XY stage 44 in the yawing direction is about 0.5",so that an error in measurement of the square's inclination or tilt of 0.5" will occur. When the error in inclination measurements of 0.5" occurs, the amount of the difference between patterns as shown in FIG. 7 will be 100 mm.times.tan 0.5".apprxeq.0.24 .mu.m in the case of a step movement of 100 mm.